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A cache stampede happens when concurrent requests all encounter the same missing or expired value and trigger overlapping work to rebuild or revalidate it. Two useful controls solve different parts of the problem: stale-while-revalidate can return an acceptable old response while a refresh runs, while request coalescing lets concurrent callers share one forward fetch when the resulting response is reusable. Use stale serving only within an explicit freshness tolerance; use coalescing to reduce duplicate work, including on some cold misses.
What causes a cache stampede?
When a popular cache entry expires or is absent, many requests can arrive before a replacement is ready. If each request independently calls the origin or rebuilds the value, the duplicate work can overload the origin or application and delay responses. A cache stampede is this concurrent duplication, not simply the fact that a cache entry expired.
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The two mitigations have different semantics and may live at different layers. HTTP caches can collapse compatible requests; application caches can coordinate regeneration per key. Stale-while-revalidate instead changes what a cache may return during refresh: it permits an acceptable stale response for a bounded period. Neither mechanism is a universal substitute for the other.
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RFC 5861 defines stale-while-revalidate as a limited extension to the period in which a cache may serve a stale response. After the response becomes stale, a request can receive that existing response while the cache attempts background revalidation without blocking. The stale window is an upper bound, not a guarantee that a refresh succeeds or that stale content is appropriate. If no request arrives during the window, or the window closes before revalidation completes, a later request may have to wait for normal validation.
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- While fresh: the cache serves the response within its freshness lifetime.
- After freshness ends, within the permitted stale window: if policy allows, the cache may serve the old response and attempt refresh in the background.
- After the stale window: stale-while-revalidate generally no longer permits stale service; the cache must follow ordinary validation and freshness rules.
RFC 5861 gives Cache-Control: max-age=600, stale-while-revalidate=30 as an example: 600 seconds of freshness followed by up to 30 seconds during which a stale response may be served while revalidation is attempted. These are illustrative values, not a production recommendation. See the RFC 5861 definition and examples.
How request coalescing prevents duplicate fetches
HTTP request collapsing combines concurrent requests on a cache miss into one forward request when the returned response can satisfy all of them. RFC 9111 describes this behavior for HTTP caches; it does not specify an application-level singleflight lock. Requests that cannot reuse the shared response still need to be forwarded, so they may wait longer. Reusability depends on the cache key and HTTP rules such as Vary, not merely on requests arriving at the same time. See RFC 9111.
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Application-cache pattern
In an application cache, a common design is a per-key in-flight promise or mutex: the first caller starts regeneration, and concurrent callers either await that same operation or, if policy permits, receive stale data. A distributed cache may require coordination across workers or machines, rather than a lock held only in one process. These are implementation patterns, not guarantees made by HTTP caching standards.
Distributed coordination needs careful failure handling. A lease can expire while its owner is still working; owner or fencing tokens can help prevent an expired owner from overwriting newer data. Define what callers do when refresh fails, and ensure lock acquisition failures do not simply send every caller to rebuild independently. There is no universal lock algorithm or set of safe timing parameters established by the HTTP sources cited here.
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Which mitigation should you use?
| Decision | Stale-while-revalidate | Request coalescing |
|---|---|---|
| Can callers tolerate an older value? | Requires an explicit, bounded tolerance for stale content. | Callers can wait for a shared fresh result; waiting latency remains. |
| Does an old cached value need to exist? | Yes. It serves an existing stale response. | No. A cache layer that supports collapsing can also reduce duplicate work on a cold miss. |
| Main effect | Hides refresh latency for requests within the stale window. | Reduces duplicate forward work when callers can reuse the same response. |
| Main constraint | Stale content may be unacceptable or prohibited, and the window is bounded. | Cache keys and response-reuse rules determine which requests can share the result. |
These controls can be combined: serve stale to avoid blocking eligible callers, while coalescing the background refresh so concurrent refresh triggers do not multiply origin work. If stale data is not safe and callers can tolerate waiting, coalescing alone may be the better fit. If you have no stale entry but want to avoid a cold-miss herd, stale-while-revalidate cannot help that miss.
When stale content is not allowed
Stale data is a correctness decision as well as a performance trade-off. RFC 9111 prohibits stale reuse where explicit directives forbid it and otherwise requires either disconnection from the origin or explicit permission. In particular, must-revalidate means the cache must successfully validate a response before reusing it. Do not treat a stale window as permission to ignore protocol directives or application requirements such as current account balances, inventory, access rights, or other rapidly changing sensitive values.
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Stale-while-revalidate versus stale-if-error
stale-while-revalidate addresses refresh latency: stale content may be served while a background revalidation is attempted. stale-if-error addresses origin failure: stale content may be used when an error would otherwise be returned. RFC 5861 defines them as independent extensions, so configure and reason about each separately; one does not imply the other. Neither overrides business correctness requirements. The definitions are in RFC 5861 and the general stale-response rules in RFC 9111.
CloudFront example: check effective TTL policy
AWS documents Cache-Control: max-age=3600, stale-while-revalidate=600 as a CloudFront example: one hour of freshness, followed by up to ten minutes of stale-while-revalidate availability. For CloudFront, the effective stale allowance is limited by the lesser of the directive and the distribution’s configured maximum TTL. AWS also documents a combined example with stale-if-error=86400; that is a platform-specific example, not a universal default. See AWS’s CloudFront expiration and stale-content guidance.
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Do not assume origin headers alone determine what CloudFront caches. Its cache policy sets cache-key inputs and minimum, default, and maximum TTLs. AWS warns that a minimum TTL greater than zero can cause caching for at least that duration even when an origin response includes no-cache, no-store, or private. Review the actual policy alongside origin headers and content sensitivity in the CloudFront CachePolicyConfig reference.
AWS separately documents CloudFront behavior for expired cached objects when an origin returns a 5xx response, as well as stale directives for specifying stale availability. These are CloudFront behaviors; do not assume another CDN or application cache behaves the same way. See CloudFront handling of origin 4xx and 5xx responses.
Quick Recap
Implementation checklist
- Decide which responses may be served stale, and define the maximum acceptable stale interval for each class of content.
- Choose the layer for each control: HTTP cache directives and CDN policy for edge behavior; per-key in-flight coordination for application regeneration.
- For coalescing, verify which callers share a cache key and representation, including relevant
Varydimensions. - Set stale windows and ordinary freshness lifetimes deliberately; the RFC and CloudFront numbers above are examples, not measured performance results or recommended defaults.
- Check directives such as
must-revalidate, origin failure behavior, and any business rule that forbids stale data. - For CloudFront, inspect the effective minimum, default, and maximum TTLs and cache-key configuration, not just the origin’s
Cache-Controlheader. - For distributed refresh coordination, define lease ownership, expiry, stale-owner protection, refresh failure behavior, and what happens if coordination itself fails.
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